Heart failure (HF) is a condition that afflicts increasingly more people each year. This condition is defined as the common end-stage of many frequent cardiac diseases (e.g. myocardial infarction, pressure overload, volume overload, viral myocarditis, toxic cardiomyopathy), and is characterized by relentless progression. The resultant myocardial damage from such events in conjunction with neurohormonal and cytokine activation, is suspect for the causes of chamber remodeling of the heart, an initial phase of HF. Early diagnosis of HF is difficult because the remodeling process precedes the development of symptoms by months or even years. The current diagnostic tests (e.g. two dimensional echocardiogram coupled with Doppler flow studies) only reveal changes in the heart in the late stages of the disease. To date, no cure for HF exists. Early diagnosis is a key factor in achieving a good prognosis and management of this disease.
An imaging agent that identifies patients in early HF would enable immediate treatment and life-style improvements for those living with this disease. In the past, researchers have investigated a variety of biological markers found in HF to develop methods for detection of early stages of HF. The cardiac sympathetic nervous system (CSNS), which is part of the autonomic nervous system, was found to be one of the biological markers of interest.
The autonomic nerve system, which plays a crucial role in regulating cardiac function, consists of the CSNS and the cardiac parasympathetic nervous system (CPNS). In the two branches of the cardiac autonomic innervations, the CSNS and CPNS, postganglionic sympathetic neurons communicate with each other via the neurotransmitter norepinephrine (NE). These branches work in finely tuned opposition to each other in the heart. Thus stimulus to the sympathetic nerve system causes increased contractility, acceleration of heart rate and conduction, which is mediated by the action of NE on post synaptic β1 adrenoceptors. Stimulation of the parasympathetic nerves on the other hand, leads to a decrease in heart rate and conduction. This is mediated by action of acetylcholine on postsynaptic M2 muscarinic acetylcholine receptors.
NE is the neurotransmitter of postganglionic sympathetic neurons. NE is stored in vesicles within the neurons and is released by Ca+2 mediated exocytosis into the synaptic cleft upon nerve depolarization. Most of the norepinephrine released is returned to the neuron by the norepinephrine transporter (NET; also known as “Uptake-1” mechanism) and repackaged into storage vesicles by the vesicular monoamine transporter (VMAT). The remaining amount of NE in the synaptic cleft binds to postsynaptic 131 adrenoceptors controlling heart contractility, acceleration of heart rate and heart conduction. Tissue concentrations of NE in the normal heart are generally considered to be reliable markers of regional sympathetic nerve density, which are uniformly distributed throughout the heart.
Abnormalities in cardiac innervation have been implicated in the pathophysiology of many heart diseases, including sudden cardiac death, congestive heart failure, diabetic autonomic neuropathy, myocardial ischemia and cardiac arrhythmias. Heart failure is characterized by a hyperadrenergic state whereby increased systemic levels of NE and increased local spillover of catecholamines occurs. It has been documented that there is a reduction in cardiac uptake-1 density or function in tissue samples of both human patients and animal models, which may be the reason for the increased amount of systemic NE observed in myocardium tissue. Development of methods to assess physiological changes of NE uptake-1 in the myocardium are therefore highly desirable.
As disclosed in United States Patent Application Publication No. 20060127309 (herein incorporated by reference in its entirety), medical radionuclide imaging (e.g., Nuclear Medicine) is a key component of modern medical practice. This methodology involves the administration, typically by injection, of tracer amounts of a radioactive substance (e.g., radiotracer agents, radiotherapeutic agents, and radiopharmaceutical agents), which subsequently localize in the body in a manner dependent on the physiologic function of the organ or tissue system being studied. The radiotracer emissions, most commonly gamma photons, are imaged with a detector outside the body, creating a map of the radiotracer distribution within the body. When interpreted by an appropriately trained physician, these images provide information of great value in the clinical diagnosis and treatment of disease. Typical applications of this technology include detection of coronary artery disease (e.g., thallium scanning) and the detection of cancerous involvement of bones (e.g., bone scanning). The overwhelming bulk of clinical radionuclide imaging is performed using gamma emitting radiotracers and detectors known as “gamma cameras.”
Recent advances in diagnostic imaging, such as magnetic resonance imaging (MRI), computerized tomography (CT), single photon emission computerized tomography (SPECT), and positron emission tomography (PET) have made a significant impact in cardiology, neurology, oncology, and radiology. Although these diagnostic methods employ different techniques and yield different types of anatomic and functional information, this information is often complementary in the diagnostic process. Generally speaking, PET uses imaging agents labeled with the positron-emitters such as 18F, 11C, 13N and 15O, 75Br, 76Br and 124I. SPECT uses imaging agents labeled with the single-photon-emitters such as 201Tl, 99Tc, 123I, and 131I.
Glucose-based and amino acid-based compounds have also been used as imaging agents. Amino acid-based compounds are more useful in analyzing tumor cells, due to their faster uptake and incorporation into protein synthesis. Of the amino acid-based compounds, 11C- and 18F-containing compounds have been used with success. 11C-containing radiolabeled amino acids suitable for imaging include, for example, L-[1-11C]leucine, L-[1-11C]tyrosine, L-[methyl-11C]methionine and L-[1-11C]methionine.
PET scans involve the detection of gamma rays in the form of annihilation photons from short-lived positron emitting radioactive isotopes including, but not limited to 18F with a half-life of approximately 110 minutes, 11C with a half-life of approximately 20 minutes, 13N with a half-life of approximately 10 minutes and 15O with a half-life of approximately 2 minutes, using the coincidence method. For PET imaging studies of cardiac sympathetic innervation, carbon-11 (11C) labeled compounds such as [11C]meta-hydroxyephedrine (HED) are frequently used at major PET centers that have in-house cyclotrons and radiochemistry facilities. Recently the nuclear medicine market has seen a substantial increase in stand-alone PET imaging centers that do not have cyclotrons. These satellite-type facilities typically use 2-[18F]fluoro-2-deoxy-D-glucose (FDG) for PET imaging of cancerous tumors.
SPECT, on the other hand, uses longer-lived isotopes including but not limited to 99mTc with a half-life of approximately 6 hours and 201Tl with a half-life of approximately 74 hours. The resolution in present SPECT systems, however, is lower than that presently available in PET systems.
Radiotracers targeting each branch of cardiac autonomic innervation have been developed. The number of tracers developed for the sympathetic neurons however is far more than those developed for the parasympathetic neurons. There are two reasons for this. First, the NET is nonselective and will readily transport structural analogs of NE into the sympathetic varicosity. The choline uptake carrier on the other hand is highly selective. Second, there is a dense population of the sympathetic nerves in the left ventricular wall as compared to the parasympathetic neurons found in the thin walls of the atria and conduction nodes. This has therefore, made imaging the sympathetic neurons easier. The structures below are examples of radiolabel led catecholamines and catecholamine analogues, and guanadines used for studying cardiac sympathetic neurons.
Radiolabelled Catecholamines and Catecholamine Analogues, and Guanidines Used for Studying Cardiac Sympathetic Neurons

[11C]Dopamine ([11C]DA) and 6-[18F]fluorodopamine (6-[18F]FDA) have been used to image dogs and baboons respectively. 6-[18F]FDA showed rapid uptake and clearance, and good images of the heart. [11C]Norepinephrine ([11C]NE) has been used to obtain planar images of canine heart and clearly visualized the left ventricular myocardium in a cynomologous monkey. 6-[18F]Fluoronorepinephrine (6-[18F]FNE) has also been used to image the baboon heart and showed high uptake and retention. Myocardial kinetics of [11C]epinephrine ([11C]EPI) has been extensively studied and is handled in a similar manner to NE and has been used to assess neuronal changes in cardiac transplant patients.
The catecholamine analogues like 1R,2S-6-[18F]-fluorometaraminol (6-[18F]FMR), [11C]hydroxyephedrine ([11C]HED) and [11C]phenylephrine ([11C]PHEN) have also been used very effectively to study the sympathetic nerve system. [123I]-meta-Iodabenzylguanidine (MIBG) is another extensively studied catecholamine analog that shows neuronal uptake as well as uptake by the cardiac myocytes, when studying sympathetic nerve fibers of the heart. Studies with MIBG allow clinicians to map the regional distribution of nerve fibers in the heart using imaging devices found in all nuclear medicine clinics. MIBG is also used for diagnostic imaging and radiotherapy of adrenergic tumors, such as neuroblastoma and pheochromocytoma. [123I]-MIBG has been used to delineate nerve damage while [11C]HED has been used to demonstrate neuronal abnormalities in a number of heart conditions including transplanted hearts, cardiomyopathy, acute myocardial infarction and cardiac diabetic neuropathy. MIBG is a SPECT tracer, however, and therefore does not provide quantitative information.
Lastly, [125I]-CAAP was the first 125I-radiolabeled 1-carboxamidino-4-phenyl-piperazine. Comparison studies of [125I]-CAAP with [125I]-MIBG in tissue distribution studies in rats demonstrated equivalent uptake of the radiotracer in heart tissue. The uptake and retention of the compounds in the myocardium tissue are speculated to be due to the same mechanism of action, which recognizes the guanidine functionality in both substrates. NET uptake-1 is a possible mode of action. Several positron emitting radiotracers were therefore developed as shown below.
MIBG and Positron Emitting Analogues

Of the three benzylguanidine PET tracers developed only one, 4-[18F]fluoro-3-iodobenzylguanidine ([18F]FIBG) demonstrated uptake and behavior similar to MMG in vivo.
All the tracers mentioned above give valuable information but have their limitations. These include metabolic instability (NE, FNE, DA, FDA, PHEN, EPI and CAAP) or pharmacologically active norepinephrine release (FMR). MMG also has its drawbacks. It has considerable extraneuronal uptake mediated by passive diffusion and by the uptake-2 (membrane transport) mechanism. And, being a SPECT agent, like CAAP, MIBG does not give quantitative information and has other associated limitations. There is therefore a need for tracers that will show the following characteristics:
a) stability,
b) not cause NE release (thereby reducing side effects),
c) give quantitative information, and/or
d) high affinity for VMAT.